Too much pitch???

erniewillis

Recruit
Joined
Oct 2, 2018
Messages
5
I have a 1987 Bass Buggy Pontoon that I restored for my wife. It was her dads and he left it to her before he died. Her dad had a 1990 Johnson 45 H.P. commercial engine and ran a 13.75 x 15 prop for over 10 years with no issues. I did reconfigure the seats during the resto but have less weight now than before due to seat bases and supports being plastic instead of 3/4 plywood. Problem is same engine,same boat and I get a terrible prop spin. It is not a blown hub as I have already replaced it. Do I need to drop the pitch down to a 13 or even 11?
 

Bikestdy

Seaman Apprentice
Joined
Feb 20, 2018
Messages
39
Are you saying that the engine over revs? Or is it cavitating? If it is over revving, you would go up with pitch, but would need to know how high out is revving to figure out what pitch to go to. Also where is the engine mounted at the correct height? Where is the ventilation plate when running, in relation to the water? I would first be looking at engine height and how much weight as reduced?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
A pity that engine was not previously tached over the years to know max wot rpm range achieved as loaded to know if engine run min-max wot rpm range factory stated. Check if the hub was installed right, was it torqued to specs ? if not prop is over revving on prop shaft.

If that's the issue, once corrected make a new wot run and report max achieved rpm, once knowing can go for a prop maximization to a less pitch if in need to pull wot revs up. Will need a tach for that if wanting to get the max out of that engine.

Happy Boating
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
2 thoughts.
1st, 15" pitch props are a LOT of pitch on a 45hp powered 'toon. That by itself is not likely the source of your headache though.

2nd, if the boat is lighter, or you've raised the back of the boat any with weight distribution changes, the engine may not be sitting in the water as deed as it was originally. This is where my focus would be. If you can, maybe try dropping it a couple of inches to see what that does for your problem. If that cures it you can work from there.

Last thought, there's nothing hanging down into the water ahead of the engine is there? ANYTHING that might be screwing up the water flow to the front of that prop needs to be eliminated....
 

erniewillis

Recruit
Joined
Oct 2, 2018
Messages
5
The engine is in the same spot as original,which is as low as it will mount. The ventilation plate is below the waterline. Everyone seems to want to sit in the front couches when running but I will move everyone to the back saturday just to see if it is a weight distribution issue. It had a standard aluminum prop,no cupping and I had a new hub installed but the issue remained. I bought a new stainless prop (same size) and it has some cup in it but it too has the same issue.

I may not be using the correct terminology here. All I know is that I can not get to WOT because it acts like I hit a patch of ice after so much throttle. Lets face it, that 45 Commercial Johnson aint that strong. Sometimes it will start spinning in turns but I think it is more relevant to my throttle position. I can be running along at 1/4-1/3 throttle on smooth water and all of a sudden the engine revs like crazy and I ain't moving accordingly.
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
If it's doing it when you turn as well as when it gets to a certain rpm, in my experience it's a pretty safe bet the engine is too high. Something needs to be done to get that prop down into the water further.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Is there a tach installed to check the rpm behaviour when engine runs OK and when misbehaves by over revving soon after ? Toons that doesn't plane, just achieves fast displacement speeds must run parallel to water level, with deck weight evenly distributed and engine trimmed to 90 deg for top prop thrust. Test that way on flat calm water cond and report your findings.

If after testing as suggested and asuming your engine and prop runs strong without any tech issues, seems the engine sits way up on transom, will need to find a form to lower it from where now sits.

If without a tach won't be able to install a spot on prop for your application and load. Would advise to install one as the post doesn't specify any wot rpm numbers whatsoever.

Happy Boating
 
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